Excitonic transistor switches optical signals

PORTLAND, Ore.  Expensive optical-to-electronic-to-optical converters are needed to switch photonic signals. Now, researchers have demonstrated an excitonic transistor that can perform the operation in what they claim is an ultra-small, ultra-low-power and potentially ultra-cheap device.

According to researchers at the University of California at San Diego, the excitonic transistor accepts optical signals at its input, converts them to excitons which can be switched electronically, then automatically regenerates the optical signal at its output through recombination.

"We have demonstrated the simplest possible way to perform a very natural interconnect between light and electronics. Excitons store light, and at the same time can be controlled electronically," said Alex High, a doctoral candidate UC-San Diego. "We have already shown that they can be integrated into small, fast devices with performance that is on par with traditional modulators and at very low power levels, since they don't draw any active current except when you switch them."

Excitons are electron/hole pairs formed when a laser excites an electron to jump from the valence band (where it is bound to a semiconductor atom) to the conduction band (where it is free to move about) leaving behind a hole. The researchers created their excitons on a GaAs chip adjacent to two quantum wells--one for storing the electrons and the other for storing the holes

A circuit that uses excitons for computing produces light as the particles recombine to release photons.

"Our excitons were created with a 682-nanometer wavelength helium-neon laser near two quantum wells embedded inside a gallium arsenide chip," said High. "When photons hit the electrons in the chip, they jump from the valence band to the conduction band leaving a hole behind, but since they are oppositely charged, they are attracted to each other to form a bound state--the same way a proton binds an electron to it to form a hydrogen atom. We then trap the electrons in one quantum well and the holes in an adjacent quantum well, by virtue of their [voltage] potentials."

A pair of 8-nm-wide quantum wells were separated by just 4 nanometers--the size of about 20 silicon atoms. Without the quantum wells, the excitons would have a very short lifetime before recombining, releasing their energy as a photon. They are trapped in separate quantum wells they can persist for as long as a microsecond--long enough for thousands of operations to be performed by them.

The researchers demonstrated how the excitons in the GaAs chip could perform calculations by arranging them into circuits governed by the same voltage potentials that make normal transistors work. The exciton transistor has a semiconductor source and drain like a normal field-effect transistor, and are controlled by a conventional metal gate.

"We put our transistor source at an elevated [voltage] potential and the drain at a lower potential, with an intermediate metal gate which acts as a barrier whose potential controls the flow of excitons through the transistor channel from the higher- to lower-potential--very similar in principle to the operation of a traditional field-effect transistor," said High. "The excitonic transistor consists of a stacked structure--on the bottom is a ground plane, with the two quantum wells layered on top of that--then a layer of regular gallium arsenide and on top of the stack we fabricated metal gates which create electric fields in the quantum wells to control the excitons."

The circuit consisted of three exciton transistors wired as an analog adder, whereby one output transistor combined the signals from the input transistors in a summation operation with 95 percent accuracy, according to High. The excitons exiting the summing transistor were then allowed to recombine, producing a photon, thereby converting the electronic signal back into light.

The group showed they could tune the wavelength of light over a range of about 40 nanometers. In the demonstration, the emission wavelength was about 790 nanometers, but in principle it could be lengthened to traditional communications frequencies like 1,550 nanometers.

Next, the researchers said they plan to improve the switching time, which is currently 200 picoseconds, and integrate their circuits into more complicated logic circuits. By taking advantage of an excitonic memory element that can hold values for up to a microsecond, mathematical calculations could be performed.

The group will demonstrate other normal transistor functions with its excitonic transistors. "We also plan to make excitonic transistors that perform functions like amplification rather than just switching," said High.

They also plan to fabricate room-temperature devices that would eliminate the need with current devices for cooling to 40 degrees Kelvin (minus 390 degrees Fahrenheit).

Research funded was provided by the U.S. Army Research Office, the U.S. Energy Department and the National Science Foundation.